Glacier Remote Sensing Using Sentinel-2. Part I: Radiometric and Geometric Performance, and Application to Ice Velocity
"> Figure 1
<p>Sentinel-2 bands in comparison to Landsat 8. The coloured area indicates the wavelength range where the atmosphere is transparent. The vertical placement of the rectangles indicating the individual bands is arbitrary.</p> "> Figure 2
<p>Sentinel-2A swath pattern and colour-coded number of relative orbits from which a point on the ground is seen. <b>Upper panel</b>: global pattern; <b>lower panel</b>: detail over Europe. In the south of Norway, for instance, every ground point is seen from two different relative orbits, while in the very north of Norway it is from three orbits, increasing the temporal resolution of Sentinel-2A.</p> "> Figure 3
<p>Sections of about 300 by 300 pixels over dark water in Sentinel-2A bands 4, 8, and 11 with enhanced histograms. Both along-track (vertical) and cross-track (horizontal) stripes become visible over such dark surfaces. The upper curves indicate the along-track column means in digital numbers (DN). Stripes amount on average between ±7 DN and ±4 DN (0.0007 and 0.0004 TOA reflectance) for bands 4 and 8, and 11, respectively.</p> "> Figure 4
<p>Dark shadow in a mountain flank in the Karakoram. (<b>a</b>) Overview with location of other panels marked by a white square of 1 km × 1 km in size; (<b>b</b>) Section of a Sentinel-2A image (30 November 2015); (<b>c</b>) Landsat 7 ETM+ (17 November 2001); (<b>d</b>) Landsat 8 (2 December 2015). All examples are using band 8 of the respective sensors with enhanced histogram. North to the top. Note the crevasses to the upper middle of the Sentinel-2A and Landsat 8 images.</p> "> Figure 5
<p>Vertical errors <span class="html-italic">Δh</span> in a DEM used for orthoprojection of satellite scenes translate into horizontal orthoimage offsets from the true location of point <span class="html-italic">P</span>. These cross-track offsets depend on the magnitude of <span class="html-italic">Δh</span> and the off-nadir cross-track look angle of the sensor towards point <span class="html-italic">P</span>. Compared to true ground, coordinates of point <span class="html-italic">P</span> the horizontal offsets <span class="html-italic">P–P<sub>i</sub>, P–P<sub>j</sub>,</span> or <span class="html-italic">P–P<sub>k</sub></span> become effective, while the offsets <span class="html-italic">P<sub>i</sub>–P<sub>j</sub></span>, or <span class="html-italic">P<sub>i</sub>–P<sub>k</sub></span> appear when comparing two orthoimages from different orbits. View in orbit plane.</p> "> Figure 6
<p>Cross-track component of co-registration offsets between Sentinel-2A data from the same orbit, R120, from 28 November 2015 and 8 December 2015. UTM-tile T40RGS, 110 × 110 km in size. North is to the top and the five slightly oblique blocks of about 20 km width indicate the orbit direction from north to south (arrow). There is an overall shift between the two scenes of about 8.5 m, and offsets between the pushbroom modules are up to about 1–2 m. Coordinate grid: UTM zone 40N.</p> "> Figure 7
<p>(<b>a</b>–<b>c</b>) Along-track component (middle panel, (<b>b</b>)) and cross-track component (right panel, (<b>c</b>)) of co-registration offsets between Sentinel-2A data from the same orbit, R120, from 30 August 2015 and 9 September 2015 (commissioning phase), showing jitter. Complete UTM-tile T41SKS, border region between Iran and Afghanistan (<b>a</b>). Arrow indicates flight direction.</p> "> Figure 8
<p>Cross-track components of Landsat co-registrations. Path/row 39/36, Kelso area, Mojave, California (<b>a</b>); Middle panel (<b>b</b>): Landsat 7 band 8 data of day 337 and 353 of 2002; right panel (<b>c</b>) Landsat 8 band 8 data day 359 of 2013 and day 10 of 2014. Arrow indicates flight direction.</p> "> Figure 9
<p>(<b>a</b>) Elevation differences between a DEM from the Norwegian mapping agency and a DEM from <a href="http://www.viewfinderpanoramas.org" target="_blank">www.viewfinderpanoramas.org</a> based on Soviet maps; (<b>b</b>) cross-track offsets between two Sentinel-2A scenes from 18 August and 22 August 2015, UTM-tile T33WXT, Lyngen, Northern Norway. The cross-track offsets (right) are scaled to reconstruct the DEM errors that lead to them. The pattern of both panels is similar, besides typical matching errors. Coordinate grid: UTM zone 35N.</p> "> Figure 10
<p>(<b>a</b>) Map of stable areas (red) in West Greenland near Jacobshavn Isbræ (Sermeq Kujalleq) used for calculating co-registration accuracy; background image: Sentinel-2, band 8, 16 August 2015; (<b>b</b>,<b>c</b>) histograms of co-location accuracy in Easting and Northing of Sentinel-2 band 8. The red curves in (<b>b</b>,<b>c</b>) are Gaussians with the same mean and standard deviation as calculated from the co-location offsets. For coordinates see <a href="#sec4dot3-remotesensing-08-00598" class="html-sec">Section 4.3</a>.</p> "> Figure 11
<p>Sentinel-2A cross-track offsets between two neighbouring swaths. Scenes from 29 September (orbit R131) and 6 December 2015 (R088), UTM-tile T52KHU, Northern Territory, Australia. Coordinates in UTM zone 52.</p> "> Figure 12
<p>(<b>a</b>,<b>b</b>) Cross-track offsets (<b>b</b>) between sections of a Landsat 8 and a Sentinel-2A (<b>a</b>) scene from 8 September 2015 (10:10 and 10:30 UTC, respectively) over Zermatt, Gorner Glacier and Findelen Glacier, Swiss Alps. Data voids in the offset field are due to mismatches over clouds. Colour-coded offsets underlain by a DEM hillshade. Distinct offsets over the glaciers are due to glacier thickness loss between the date of the DEM used for ortho-rectification and the image acquisition in 2015.</p> "> Figure 13
<p>(<b>a</b>) Ice velocities between Sentinel-2 data from 30 July (background) and 8 September 2015; (<b>b</b>) Ice velocities between Landsat 8 data from 7 August (background) and 8 September 2015; (<b>c</b>) Velocities between Landsat 8 data from 7 August and Sentinel-2 data from 8 September (background). Maximum speeds ~0.8 m/d. The same matching windows (in ground size) and the same threshold for correlation coefficients have been used. Outliers have not been filtered manually. Coordinates: UTM zone 32N.</p> "> Figure 14
<p>(<b>a</b>) Ice speeds on Fox Glacier, New Zealand, from Sentinel-2A data from 24 December 2015 and 3 January 2016; (<b>b</b>) Speed differences between 24 December 2015 and 3 January 2016, and 3 January 2016 and 13 January 2016 (calculated as time 2 minus time 1 so that positive differences indicate increase in speed). Outliers have been removed based on low correlation values and residuals of the vector sum of the two 10-day displacements and the full 20-day displacements. White glacier outlines from [<a href="#B33-remotesensing-08-00598" class="html-bibr">33</a>]. Coordinates in UTM zone 59S.</p> "> Figure 15
<p>Sentinel-2A based ice velocity map at west coast of Greenland (<b>a</b>). Black lines (1) and (2) indicate the velocity profiles, shown in <a href="#remotesensing-08-00598-f016" class="html-fig">Figure 16</a>; (<b>b</b>) frequency-scatterplot of velocity in east and north direction for non-moving areas after adjustment by a constant co-location offset (colour code: from blue to red indicates higher frequency). The arrow indicates the flight direction.</p> "> Figure 16
<p>Comparison of ice speed along central flowlines of the glaciers Sermeq Kujalleq (profile 1) and Sermeq Avannarleq (profile 2) from Sentinel-2A data from 16 August and 8 September 2015 and a mean Sentinel-1A based ice velocity map [<a href="#B36-remotesensing-08-00598" class="html-bibr">36</a>].</p> "> Figure 17
<p>Ice speeds on a section of the Antarctic Peninsula (Larsen C ice shelf to the right) from Sentinel-2A data from 8 and 18 January 2016. White areas are clouds in either of the images automatically detected and combined from band 11/band 8 ratios because the default Sentinel-2 L1C cloud masks failed. Displacements with low correlation values (due to clouds, fog, lack of contrast, features larger than matching window used) have been masked out. The correlation window size suitable for the outlet glaciers is not optimal for the ice shelf. The sub-panel to the right shows displacement results over a section of the ice shelf (black rectangle) using a larger window size than used for the main panel.</p> ">
Abstract
:1. Introduction
- Four visible and near-infrared (VNIR) bands with 10 m spatial resolution, compared to 30 m (15 m for pan) for Landsat 8 OLI (Figure 1);
- Six VNIR and short-wave infrared bands (SWIR) with 20 m resolution, compared to 30 m for Landsat 8 OLI;
- The Sentinel-2 MSI swath width is 290 km against the 185 km of Landsat 8 (at the cost of larger off-nadir viewing angles and thus larger potential ortho-rectification errors; see Section 3);
- The Sentinel-2A orbit repeat rate is 10 days against 16 days of Landsat 8, and will become five days from the same relative orbit after the launch of Sentinel-2B. The actual frequency of repeat acquisitions however depends on the capacity of the entire system and the acquisition plan. For higher latitudes where the swaths from neighbouring orbits overlap, the potential revisit time will also be shorter than five or 10 days (Figure 2).
- It should also be noted that Sentinel-2 carries no thermal instrument, in contrast to Landsat 8.
2. Radiometric Noise and Patterns
2.1. Performance over Homogenous Surfaces
2.2. Performance in Shadows
3. Geometric Performance and DEM Effects
- (i)
- The relative geo-locational precision between different images, also called co-registration accuracy. This group of errors can be random (i.e., noise) but also contain systematic patterns such as attitude jitter or calibration errors. (The latter error patterns could also be seen as higher-order components of the following error category, ii.)
- (ii)
- Mainly shifts, but also rotation or deformation, apply to entire scenes and are scene-specific or system-specific geo-location biases in the image data with respect to the true ground location of the measurements. Typically, these biases stem from errors or inaccuracies in spacecraft attitude or position measurements or in the subsequent solution of the image orientation parameters.
- (iii)
- Of large practical significance for glacier and high-mountain applications, vertical errors in a DEM elevation used for ortho-rectification or terrain correction of the raw data propagate into a pattern of local horizontal off-nadir offsets in the ortho-rectified products such as Landsat L1T or Sentinel-2 L1C. The effect of these elevation errors depends on the off-nadir view angle, in particular in cross-track direction, and the magnitude of the elevation error (Figure 5). The maximum off-nadir distance d of a point in a Sentinel-2 scene can be 145 km (i.e., half the swath width) so that a vertical DEM error Δh translates in the worst case into a horizontal geo-reference offset in cross-track direction of
3.1. Co-Registration of Data from Repeat Orbits
3.2. Co-Registration of Data from Neighbouring Orbits
3.3. Co-Registration between Sentinel-2A and Landsat 8 Data
3.4. Co-Registration to Reference Images
4. Ice Velocity Measurement
4.1. European Alps
4.2. New Zealand
4.3. Greenland
4.4. Antarctic Peninsula
5. Conclusions
Acknowledgments
Author Contributions
Conflicts of Interest
Abbreviations
DEM | Digital elevation model |
DN | Digital number |
ETM | Enhanced thematic mapper |
InSAR | Interferometric synthetic aperture radar |
MSI | Multispectral instrument |
SWIR | Short-wave infrared |
TOA | Top of atmosphere |
OLI | Operational land imager |
VNIR | Visible and near infrared |
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Kääb, A.; Winsvold, S.H.; Altena, B.; Nuth, C.; Nagler, T.; Wuite, J. Glacier Remote Sensing Using Sentinel-2. Part I: Radiometric and Geometric Performance, and Application to Ice Velocity. Remote Sens. 2016, 8, 598. https://doi.org/10.3390/rs8070598
Kääb A, Winsvold SH, Altena B, Nuth C, Nagler T, Wuite J. Glacier Remote Sensing Using Sentinel-2. Part I: Radiometric and Geometric Performance, and Application to Ice Velocity. Remote Sensing. 2016; 8(7):598. https://doi.org/10.3390/rs8070598
Chicago/Turabian StyleKääb, Andreas, Solveig H. Winsvold, Bas Altena, Christopher Nuth, Thomas Nagler, and Jan Wuite. 2016. "Glacier Remote Sensing Using Sentinel-2. Part I: Radiometric and Geometric Performance, and Application to Ice Velocity" Remote Sensing 8, no. 7: 598. https://doi.org/10.3390/rs8070598